The present invention is directed to optical components for use in fiber optic networks and particularly to devices known as optical polarization maintaining splitters (dividers) or combiners (multiplexers).
In fiber optical transmission systems the light beams traveling in two or more fibers must often be combined into a single fiber, a device which accomplishes this is called a combiner or multiplexer. Similarly, in such systems one beam must frequently be split into two or more beams, a device which accomplishes this is called a splitter or divider. A splitter or combiner is simply the same device used xe2x80x9cin reversexe2x80x9d i.e. when a beam is launched from a single fiber through a coupler the beam will be split into multiple beams and directed to multiple output fibers, if multiple beams are launched from the previous xe2x80x9coutputxe2x80x9d (now input) fibers back through the coupler the beams will be combined into a single beams and directed to the single xe2x80x9cinputxe2x80x9d (now output) fiber as such splitter/combiners may be referred to as an optical xe2x80x9ccouplerxe2x80x9d.
A coupler commonly used in fiber optics applications is a xe2x80x9cfused couplerxe2x80x9d which is made by fusing two or more optical fibers together. These couplers are manufactured by twisting two or more optical fibers together, heating them and then drawing the fibers which fuses the multiple fibers into one. However, these couplers do not maintain polarization, thus rendering them useless for many applications as many of the components in an optical communication system utilize the polarization of the beams to operate. Accordingly, couplers used in such applications must maintain the input polarization as the beams travel through the coupler. Recently a polarization maintaining (1xc3x972) fused coupler has become available, but this device is quite expensive and is not available in other than a 1xc3x972 configuration. Many applications require 1xc3x97N PM couplers, which can be made from 1xc3x972 couplers cascaded together, but which would be prohibitively expensive and suffer a penalty in performance due to the cascading of the devices. The present invention is directed to providing multiple port 1xc3x97N or 2xc3x97N polarization maintaining couplers that are compact, efficient, low loss, high performance in extinction ratio and economical.
A polarization maintaining (PM) fiber optic coupler (1xc3x97N or 2xc3x97N) is disclosed for use in fiber optic communications. This coupler can be used as a splitter or a combiner of polarized optical signals while maintaining the degree of polarization in both cases. The coupler is based on the use of partial optical reflectors (in predetermined reflectances) in a miniaturized free space format to achieve splitting or combining of polarized optical signals. Lenses are used at the input and output ends to couple the signal into or out of polarization maintaining optical fibers. This approach preserves the degree of polarization of incoming and outgoing optical signals since there are no depolarizing mechanisms in this approach as compared to other approaches including, but not limited to, tree branching in optical waveguides and fused optical fibers.
In this invention, low loss thin film partial optical reflectors (in predetermined reflectances) are used to split or combine polarized optical signals in miniaturized micro optic free space format. The use of partial reflectors in polarization maintaining (PM) couplers is beneficial because partial reflectors do not cause depolarization of polarized light going through them. Accordingly, a relatively large number of ports can be supported in this invention to make a 1xc3x97N or 2xc3x97N PM coupler with high polarization extinction ratio and no degradation to the extinction ratio of the incoming optical beam. Input lens(es) are used to collimate the incoming beam from the input PM fiber(s) through the reflectors and output lens(es) are used to couple the optical signal into the output PM fiber(s).
The benefits and advantages of optical couplers of the present invention are:
1. A high degree of polarization extinction is achieved and preserved;
2. There is low excess optical loss;
3. There is good uniformity of splitting ratio;
4. These couplers can be extended to unlimited number of ports while preserving a high degree of polarization;
5. Low sensitivity to wavelength;
6. Low sensitivity to temperature;
7. Compact size especially in cases of large number of ports;
8. Can be operated in single polarization format by adding one or more linear polarizers;
9. Can be integrated with an isolator by using a Faraday Rotator film in combination with polarizers;
10. Different fibers for the input/output ports can be used;
11. There is no polarization mode dispersion (PMD); and
12. Similar design and components for configurations from 1xc3x972 to 2xc3x97N.